Observation of an exotic state of water in the hydrophilic nanospace of porous coordination polymers

Porous coordination polymers (PCPs), also known as metal–organic frameworks, offer precise control over pore size, geometry, and surface chemistry, enabling exploration of confinement effects on guest molecules. Prior studies indicate two regimes of nanoconfinement for water: (1) 2–tens of nanometers, where bulk-like cores coexist with interfacial layers that alter collective behavior, and (2) sub-2 nm pores, where all molecules interact strongly with pore walls, producing markedly altered structures and dynamics. Work in hydrophobic carbon nanotubes (CNTs) has suggested proximity of gas–liquid states at ambient conditions and even the possibility of a solid–liquid critical point under nanoscale symmetry constraints. Motivated by these insights, this study targets hydrophilic 1-nm pores engineered in PCPs to stabilize high-density confined water (~1 g/cm³) and investigates whether its structure and dynamics manifest an exotic state—simultaneously showing ice-like order and liquid-like broken hydrogen bonds—without bulk analogs.

The paper reviews how confinement changes water properties depending on pore size: interfacial effects dominate in 2–tens of nm pores, while in sub-2 nm pores overlapping wall potentials govern all molecules. Hydrophobic CNTs have served as model systems, where experiments and simulations indicate modified hydrogen-bond networks at ambient conditions and predict a solid–liquid critical point in ~1 nm tubes at high pressures. In such symmetry-restricted environments, traditional constraints that forbid a solid–liquid critical point in bulk phases can be lifted, allowing continuous variation of properties between ice and liquid. These studies motivate testing whether analogous or related exotic states can be realized in well-defined hydrophilic nanospace of PCPs at near-ambient conditions.

- Materials and pore design: Synthesized a water-adsorbing PCP composed of oxime ligands and isophthalates forming a 2D square lattice (PCP-1); dehydrated/partially hydrated state referred to as PCP-2 (containing only coordinated water). The layered structure produces 1D channels (CH1, CH2) of ~1 nm diameter with 6.9 Å interlayer spacing. - Sorption measurements: H2O and D2O adsorption/desorption isotherms at 298 K using a BELSepor-3 apparatus after thorough degassing. Relative humidity (RH) controlled; rapid uptake of two “guest water” molecules per Cu at P/P0 ≈ 0.04 in addition to one tightly bound “coordinated water” per Cu. Minimal hysteresis observed. - Structural characterization: Single-crystal X-ray diffraction (SXRD) to locate oxygen positions of coordinated and guest water and map channel occupancy (CH1 filled, CH2 empty). Powder XRD and simulations used to support assignments. - Vibrational spectroscopy: Fourier-transform infrared (FTIR) microspectroscopy at ambient temperature on pellets (~100 μm thick) under controlled RH. Isotopic substitution (H2O↔D2O) used to assign OH stretching features and infer hydrogen-bond strengths via frequency shifts and deconvolution into Gaussian/Lorentzian components. - Dynamics by isotope exchange: Time-lapse IR after exposing H2O/HDO-loaded PCP-1 to D2O vapor. Monitored decay of H3 band area to extract exchange kinetics and deduce diffusivity using a 1D diffusion model. Channel length distribution measured by SEM (root-mean-square length ~0.8 μm) for diffusivity estimation; surface permeability treated as positive but unknown to set a lower bound on D. - Additional details: Chamber humidity regulation and monitoring, gas preparation (high-purity D2O), and sample handling without air exposure are described. Crystallographic coordinates deposited at CCDC.

- Confined water loading: At 298 K, PCP-1 hosts three water molecules per Cu—one coordinated strongly to Cu (coordinated water) and two additional guest water molecules—taken up sharply at P/P0 ≈ 0.04, with negligible hysteresis. - Pore occupancy and ordering: SXRD reveals periodic arrays of water oxygen atoms within CH1 (~1 nm diameter), with CH2 unoccupied. Oxygen positions indicate one coordinated water near the framework and two guest water sites (I and II) along the channel axis. - Distances and hydrogen-bond network: Measured O–O separations include 2.75(1) Å and 3.219(8) Å between coordinated and guest water neighbors; guest–guest separations within the network are ~2.87(1) Å, whereas site I–II separations of 3.33(2)–3.58(2) Å approach the upper H-bond limit. The average O–O distance of ~3.083 Å corresponds to a water density of ~1.02 g/cm³, comparable to bulk liquid water (0.997 g/cm³ at 298 K). - IR signatures of H-bonding: For PCP-2 (coordinated water only), sharp OH stretching peaks appear at ~3450 cm⁻1 (HB–OH) and ~3655 cm⁻1 (Free–OH), with expected isotopic shifts in D2O. Geometry implies each coordinated water bears one HB–OH to an isophthalate oxygen and one Free–OH pointing into the channel. - For PCP-1 (with guest water), a broad OH band (H3) decomposes into components corresponding to strong (~3202 cm⁻1), intermediate (~3390 cm⁻1), weak (~3539 cm⁻1), and non–H-bonded (~3630 cm⁻1, H4) OH. The Free–OH population in PCP-1 is comparable to that of coordinated water, indicating a significant fraction of broken H-bonds among guest waters despite positional ordering. - Hydrogen-bond statistics: The mean number of H-bonds per hydrogen atom is estimated at ~0.83—intermediate between values typical for liquid water and ice—indicating coexistence of ice-like order with liquid-like broken bonds. The fraction of broken H-bonds is about 17%. - Dynamics: Isotope-exchange IR kinetics yield an exponential decay with τ ≈ 3.8 × 10³ s. Using a 1D diffusion model and channel length ~0.8 μm, and accounting for unknown but positive surface permeability, the lower bound of diffusivity is D ≥ 0.6 × 10⁻¹⁹ m²/s at ~300 K, indicating faster structural rearrangements than in comparable solids. - Structural motif: The confined water forms an ordered, square-like arrangement reminiscent of structures predicted for solid–liquid supercritical water in ~1 nm CNTs.

The study set out to determine whether hydrophilic 1 nm pores in a designed PCP can stabilize an exotic water state that combines structural order with disrupted hydrogen bonding. SXRD demonstrates long-range positional order of water oxygens (ice-like), while IR spectroscopy reveals a substantial population of weak or non–H-bonded OH groups (liquid-like), yielding intermediate hydrogen-bond statistics and measurable diffusivity. These features do not map to any single bulk phase and align with theoretical predictions for solid–liquid supercritical water in hydrophobic ~1 nm nanotubes, where symmetry constraints permit a solid–liquid critical point and continuous variation of H-bond metrics. The observed square water motif and intermediate broken-bond fraction (≈17%) reinforce this analogy. The results imply that carefully engineered PCP nanopores can tune water’s phase-like behavior, offering a platform to probe fundamental questions about phase transitions under confinement and to exploit exotic water states in catalysis and separations.

Water confined in ~1 nm hydrophilic channels of a porous coordination polymer (PCP-1) exhibits an exotic state characterized by ice-like positional order and liquid-like broken hydrogen bonds and dynamics. Quantitative structural (O–O distances, density ~1.02 g/cm³) and spectroscopic (OH-stretch components, Free–OH population) analyses, together with isotope-exchange kinetics (τ ~3.8 × 10³ s; D ≥ 0.6 × 10⁻¹⁹ m²/s), support this dual character. The structural and dynamical properties closely resemble those predicted for solid–liquid supercritical water in ~1 nm CNTs, suggesting that PCPs provide a controllable, experimentally accessible platform to investigate confinement-enabled phase behavior, including putative solid–liquid critical points. Future work should map phase behavior versus loading, temperature, and pressure; quantify wall–water interaction strengths; and directly probe hydrogen positions and dynamics to decisively test for supercriticality. Potential applications include leveraging exotic water states to steer chemical reactions and advancing understanding of nanoconfined water in biological processes.

- Hydrogen positions are not resolved by SXRD, so H-bond assignments rely on IR spectroscopy and geometric inference. - The surface permeability of PCP-1 is unknown, so the diffusivity derived from isotope exchange is a lower bound. - While the observed properties align with predictions for solid–liquid supercritical water, the study does not conclusively demonstrate supercriticality; additional experiments to determine optimal conditions and the role of attractive wall potentials are needed. - CH2 channels remain unoccupied under the studied conditions; behavior at different loadings or pressures may differ. - Some spectroscopic peak assignments and quantitative H-bond counts depend on deconvolution models and assumed oscillator strengths.